Temperature dependence of thermal properties of ex vivo liver tissue up to ablative temperatures

Thermal properties of ex vivo bovine liver were measured as a function of temperature, by heating tissue samples in a temperature-controlled oil bath over a temperature range from about 21 degrees C to about 113 degrees C. Results evidenced temperature-dependent non-linear changes of the thermal properties, with the temperature of 100 degrees C representing a break point: the thermal properties increased with temperature up to 99 degrees C and then decreased above 100 degrees C. The rate of increase appeared dramatic between 90 degrees C and 99 degrees C, owing to the onset of vaporisation of water contained in the tissue. In particular, at 99 degrees C, the thermal conductivity reported an increase of about four times with respect to the value measured at 90 degrees C, whilst about a two-fold increase was reported for both the volumetric heat capacity and the thermal diffusivity. Temperatures higher than 100 degrees C were reached only after complete vaporisation of water contained in the tissue, resulting in about 70% loss of weight from the tissue. An overall decrease of about 71% and 63% was reported for the thermal conductivity and volumetric heat capacity, respectively, in the temperature range 101 degrees C-113 degrees C. A decrease of about 25% was reported in the measured values of the thermal diffusivity in the temperature range 101 degrees C-108 degrees C, whilst a slight increase of measured values, not statistically significant, was observed in the temperature range 108 degrees C-113 degrees C. The temperature dependent changes of the thermal parameters were modelled with non-linear regression analysis to calculate the best-fit curves interpolating measured data. The proposed regression models could be used to numerically assess the changes in the thermal properties of biological tissues at supraphysiological temperatures relevant in thermal ablation procedures, as well as their effect on the prediction of the ablation zone dimensions in computational models for treatment planning.